The Effect of a Mainshock on the Size Distribution of the Aftershocks

Abstract

A systematic decay of the aftershock rate over time is one of the most fundamental empirical laws in Earth science. However, the equally fundamental effect of a mainshock on the size distribution of subsequent earthquakes has still not been quantified today and is therefore not used in earthquake hazard assessment. We apply a stacking approach to well‐recorded earthquake sequences to extract this effect. Immediately after a mainshock, the mean size distribution of events, or b value, increases by 20–30%, considerably decreasing the chance of subsequent larger events. This increase is strongest in the immediate vicinity of the mainshock, decreasing rapidly with distance but only gradually over time. We present a model that explains these observations as a consequence of the stress changes in the surrounding area caused by the mainshocks slip. Our results have substantial implications for how seismic risk during earthquake sequences is assessed.

B‐Positive: A Robust Estimator of Aftershock Magnitude Distribution in Transiently Incomplete Catalogs

“…This study therefore does not address any potential issues of selection bias within the original data set. I also do not revisit postmainshock b ‐value increases (Gulia et al., 2018), as b ‐value increases are in the opposite direction of the expected bias, and there is no reason to doubt those b ‐value measurements.…”

“…It has also been argued that b ‐value may carry diagnostic information about the state of stress on the faults generating the earthquakes (Amitrano, 2003; Gulia et al., 2018; Scholz, 2015; Schorlemmer et al., 2005; Spada et al., 2013). This has led to the idea that b has potential for discriminating between foreshock and aftershock sequences (Gulia & Wiemer, 2019).…”

Interevent-time distribution and aftershock frequency in non-stationary induced seismicity

“…For small data sets parsimonious multivariate models should ideally be developed as well, to address the joint distribution of interevent times, distances and magnitudes 60 , which is ignored in our current approach. This should complement spatio-temporal ETAS models that can effectively be used when enough data are available.…”

Determining whether the worst earthquake has passed

“…In previous work, Gulia and Wiemer, together with co-workers, found that the b value normally rises during an aftershock sequence, which means that small earthquakes become more common 5 . In the present work, the authors noticed that, occasionally, the b value drops instead of rising, implying that big quakes increase in frequency.…”